1. Technical Field
This invention relates to a lead frame, a method for manufacturing the same and a semiconductor device, and more particularly to a lead frame having an Ag film (including an Ag-plated film) formed on a lead frame body and to be connected to a metallic wire connected to an electrode pad of a semiconductor chip, a method for manufacturing the same and a semiconductor device having the same.
2. Related Art
There are some conventional semiconductor devices including a semiconductor chip, a lead frame electrically connected to the semiconductor chip through a metallic wire and a mold resin sealing the semiconductor chip (see FIG. 1).
FIG. 1 is a sectional view of a conventional semiconductor device.
Referring to FIG. 1, a conventional semiconductor device 200 includes a lead frame 201, a semiconductor chip 203 and a mold resin 206.
The lead frame 201 includes a lead frame body 211 and an Ag film 212. The lead frame body 211 has a die pad 214 and a plurality of leads 215 arranged around the die pad 214. The lead frame body 211 may be made of Cu, an alloy containing Cu or an 42 alloy. Where the lead frame body 211 is made of the 42 alloy, a Cu film (not shown) serving as a contact layer is formed between the Ag film 212 and the lead frame body 211. The Cu film serves to improve the contact between the Ag film 212 and the lead frame body 211.
The Ag film 212 is formed on an upper surface 217A of an inner lead segment 217 of each the leads 215. The upper surface 212A of the Ag film 212 is connected to a metallic wire 204 (e.g. an Au wire) connected to the electrode pad 221 of the semiconductor chip 203. Thus, the Ag film 212 is electrically connected to the semiconductor chip 203. The Ag film 212 has high purity of Ag (99.9%). The Ag film 212 serves to prevent Cu contained in the lead frame body 211 (where the 42 alloy is used, Cu contained in the Cu film formed between the Ag film 212 and the lead frame body 211) from being deposited onto the upper surface 212A of the Ag film 212. To this end, the Ag film 212 must be so thick that Cu is not deposited onto the upper surface 212A of the Ag film 212. Concretely, the thickness of the Ag film 212 may be e.g. 4 μm to 5 μm.
The semiconductor chip 203 is bonded to the upper surface 214A of a die pad 214 using an adhesive 205 (e.g. Ag paste). In bonding the semiconductor chip 203 to the upper surface 214A of the die pad 214, the lead frame body 211 is heated (e.g. at the temperature of 175° C. to 200° C. for one hour so that the adhesive 205 is hardened). The semiconductor chip 203 has a plurality of electrode pads 221. The electrode pads 221 each is connected to the metallic wire 204. The metallic wire 204 is formed with the lead frame body 211 being heated (e.g. for 1 to 5 minutes at the temperature of 200° C. to 250° C.).
The mold resin 206 seals the semiconductor chip 203, the metallic wire 204, the die pad 214, a portion of the lead 215 and the Ag film 212. The mold resin 206 is hardened by heating (for 1 to 2 minutes at the temperature of 175° C. to 200° C.) (see, e.g. Patent Reference 1).    [Patent Reference 1] WO00/62341
FIG. 2 is a view for explaining the problem involved with the semiconductor device. Arrows in FIG. 2 indicate a moving path of Cu contained in the lead 215.
However, where the thickness of the Ag film 212 using expensive Ag is decreased (4 μm or less) in order to reduce the cost of the lead frame 201, owing to the heating while the semiconductor device 200 is manufactured (e.g. heating in hardening the adhesive 205, heating in forming the metallic wire 204, and heating in hardening the mold resin 206), as shown in FIG. 2, Cu contained in the lead 215 moves through the gaps 226 among Ag crystal grains 225 (in other words, Cu is diffused into the gap 226 among the Ag crystal grains 225) so that it will be deposited onto the upper surface 212A of the Ag film 212; thus, copper oxide 227 will be formed on the supper surface 212A of the Ag film 212 to deteriorate the contact with the metallic wire 204. This gives rise to a problem that the metallic wire 204 cannot be connected to the upper surface 212A of the Ag film 212.